For the gnocchi, wash potatoes and steam in perforated steamer basket until soft (at 100°C for 28 minutes or at 120°C for 15 minutes).
In the meantime, pluck and finely chop herb leaves, drain tomatoes and mash.
Let potatoes cool briefly, peel and press through a potato press while still hot. Knead into a dough with 50 g flour, semolina, yolk and 1 tsp salt. Divide dough in half, knead one half with herbs, the second half with pureed tomatoes and tomato paste, knead in 1-2 tablespoons flour as needed.
Shape the dough on a little flour into a roll (about 60 cm long) and cut into 30 gnocchi each. Press with a lightly floured fork on one side of the cut. Cook in a greased perforated cooking container (at 100°C for 10 minutes).
Melt butter in a small pan. Sauté sage leaves briefly in it, salt lightly and serve the sage butter with the gnocchi.